Insulin and the insulin-like growth factor multiplication stimulating activity (MSA) induce tyrosine aminotransferase and increase the rate of amino acid transport in HTC cells, an established line of rat hepatoma cells in tissue culture. Insulin also causes a time-dependent, complete, but reversible desensitization of cellular responsiveness to further insulin action. Insulin concomitantly decreases insulin receptor activity by 50%, which is not sufficient to account for the total loss of sensitivity. Thus there appears to be desensitization at a post-receptor step. The objective of this project is to study both the mechanisms by which insulin and MSA regulate cellular responsiveness to their own actions and the ways in which they interact. We will characterize the hormonal concentration dependence and specificity, and investigate the role of endocytosis and internalization, cytoskeletal elements, lysosomal degradation, and macromolecular synthesis in the desensitization phenomenon. Both the regulation of plasma membrane receptors for these two hormones and the regulation of cellular responsiveness (transaminase induction, amino acid transport, and rubidium transport) to these agents will be studied in parallel. Correlation, or dissociation, of these two aspects of hormonal regulation is important for an understanding of the cellular mechanisms of hormone action. An understanding of the regulation of hormone responsiveness may also be relevant to understanding the pathophysiology of human disease states associated with hormone resistance. Finally, these studies should provide information on the cellular actions of insulin-like growth factors.